PHYSICAL CHEMISTRY IN BRIEF

CHAP. 5: THERMOCHEMISTRY [CONTENTS] 131

5.2 Standard reaction enthalpy ∆rH◦

D Definition
The standard reaction enthalpy is the reaction heat of a chemical reaction that proceeds at constant
temperatureT(it must be stated) and standard pressurepst= 101.325 kPa.

Note:A less accurate term standard heat of reaction is often used for the standard reaction

enthalpy in the same meaning.

5.2.1 Standard enthalpy of formation ∆fH◦.

Definition:The standard enthalpy of formation is the standard reaction enthalpy of a reaction

during which one mole of a given substance is formed from elements. The elements are assumed

to enter the reaction in those phases which are the most stable at a given temperature and

standard pressure; if the elements are gaseous, they are considered in their most stable molecular

forms in the state of an ideal gas.

Note:At common temperatures, carbon exists in the crystalline modification of graphite

and not diamond; hydrogen, nitrogen and oxygen exist in the form of two-atom molecules

H 2 , N 2 , O 2.

Example

The standard reaction enthalpy of the reaction (5.9) is the standard enthalpy of formation of

methane, the standard reaction enthalpy of the reaction (5.10) is the standard enthalpy of for-

mation of carbon monoxide.

In consequence of Hess’s law, the standard reaction enthalpy of every chemical reaction

equals the difference between the standard enthalpies of formation of its products and reactants

multiplied by the respective stoichiometric coefficients. For the general reaction (5.1) we thus

have

∆rH◦ = r∆fHR◦+s∆fHS◦+ ··· −a∆fHA◦−b∆fHB◦− ···

=

∑n

i=1

νi∆fHi◦ (5.12)